We demonstrated the feasibility, and epidemiologic validity of utilizing PCR ribotyping (amplication of the multiple-allelic intergenic spacer region of the rRNA gene cluster) as a technique for investigating the relatedness of isolates of C. difficile. We have, in collaboration with the Hospital Epidemiology Service, been involved in tracking potential outbreaks of C. difficile-associated diarrhea in the Clinical Center, and envisage that this close collaboration will continue. We are now examining, with the collaborative help of Dr. Matthew Samore, Deaconess Hospital, Boston, MA and Dr. Haru Kato, Gifu University School of Medicine, Gifu, Japan, the relative utility of a number of molecular epidemiologic approaches to typing C. difficile, including PCR ribotyping, arbitrary primed-PCR (AP-PCR), pulsed-field gel electrophoresis (PFGE) and immunoblotting. We envisage not only being able to make some reasonably definitive statements about the merits, or lack thereof, of these various techniques, but also to be in possession of a highly characterized set of C. difficile isolates. Such collections of isolates are invaluable when one seeks to evaluate new techniques for strain typing.